Was to purchase 90# roll roofing the same color (or close) as the shingles being used. Cut the material the long way so you have enough 18" wide to lay upside down (granular side down) in the valley. Nail it along the edges to keep it in place. Now take enough full width (36") and lay it right side up the length of the valley. Nail it in place outside of the first layer. Be sure it is laid down completely into the valley. Now install the shingles as you normally would. I usually trim the shingles so there is a 3" spacing at the top of the valley to a 6" space at the bottom. I have used this method on several homes. One of them has 25 years on the roof this year and another has 29. The valleys still look good and have never leaked. Both are located in climates where the temperature varies from -30F in the winter (with plenty of snow) to mid 90's in the summer. Good luck!